Increased early growth rates decrease longevities of conifers in subalpine forests

For trees, fast growth rates and large size seem to be a fitness benefit because of increased competitiveness, attainment of reproductive size earlier, reduction of generation times, and increased short‐term survival chances. However, fast growth rates and large size entail reduced investment in defenses, lower wood density and mechanical strength, increased hydraulic resistance as well as problems with down‐regulation of growth during periods of stress, all of which may decrease tree longevity. In this study, we investigated the relationship between longevity and growth rates of trees and quantified effects of spatial environmental variation (elevation, slope steepness, aspect, soil depth) on tree longevity. Radial growth rates and longevities were determined from tree‐ring samples of 161 dead trees from three conifer species in subalpine forests of the Colorado Rocky Mountains (Abies lasiocarpa, Picea engelmannii) and the Swiss Alps (Picea abies). For all three species, we found an apparent tradeoff between growth rate to the age of 50 years and longevity (i.e. fast early growth is associated with decreased longevity). This association was particularly pronounced for larger P. engelmannii and P. abies, which attained canopy size, however, there were also significant effects for smaller P. engelmannii and P. abies. For the more shade‐tolerant A. lasiocarpa, tree size did not have any effect. Among the abiotic variables tested only northerly aspect significantly favored longevity of A. lasiocarpa and P. engelmannii. Trees growing on south‐facing aspects probably experience greater water deficits leading to premature tree death, and/or shorter life spans may reflect shorter fire intervals on these more xeric aspects. Empirical evidence from other studies has shown that global warming affects growth rates of trees over large spatial and temporal scales. For moist‐cool subalpine forests, we hypothesize that the higher growth rates associated with global warming may in turn result in reduced tree longevity and more rapid turnover rates.     

Regeneration response and seedling bank dynamics on a Dendroctonus rufipennis‐killed Picea engelmannii landscape

Question: How does regeneration response to a host‐specific, high‐severity, infrequent Dendroctonus rufipennis outbreak differ from our conceptualization of high‐severity, infrequent/low‐severity, frequent disturbance regimes in Picea engelmannii‐Abies lasiocarpa communities? Location: Southern Utah, USA. Methods: One hundred and seven plots across a high‐elevation P. engelmannii forest were sampled to reconstruct pre‐outbreak overstory and seedling bank densities, and calculate their associated metrics of diversity. Decade of establishment by seedling bank trees indicated “chronic” and “pulse” regenerators. Results: The post‐outbreak overstory and seedling bank were dominated by A. lasiocarpa. Although Pinus flexilis, Pinus ponderosa, Picea pungens, and Psuedotsuga menziesii were present in the overstory, they were virtually absent in the seedling bank. Seedling bank recruitment of A. lasiocarpa and P. engelmannii has been occurring chronically for at least the last ～205 and ～152 years, respectively. A pulse response of seedling bank Populus tremuloides was apparent; however, results were complicated by intense ungulate browsing. Conclusions: Despite some similarities to the high‐severity, infrequent/low‐severity, frequent conceptualization of regeneration response to disturbance, the high‐severity D. rufipennis outbreak is best described by explicitly considering host specificity and severity. Although, the outbreak simultaneously promoted both a pulse of P. tremuloides and a release of chronically regenerated A. lasiocarpa, the P. tremuloides response was generally masked by ungulate browsing, and the regeneration response came overwhelmingly from the A. lasiocarpa seedling bank. In this landscape, once dominated by P. engelmannii, the chronically regenerating seedling bank, typically thought to take advantage of canopy gaps associated with low‐severity disturbances, is poised to dominate forest reorganization in response to the host‐specific outbreak.     

Tree growth response to drought and temperature in a mountain landscape in northern Arizona, USA

Aim To understand how tree growth response to regional drought and temperature varies between tree species, elevations and forest types in a mountain landscape. Location Twenty‐one sites on an elevation gradient of 1500 m on the San Francisco Peaks, northern Arizona, USA. Methods Tree‐ring data for the years 1950–2000 for eight tree species (Abies lasiocarpa var. arizonica (Merriam) Lemm., Picea engelmannii Parry ex Engelm., Pinus aristata Engelm., Pinus edulis Engelm., Pinus flexilis James, Pinus ponderosa Dougl. ex Laws., Pseudotsuga menziesii var. glauca (Beissn.) Franco and Quercus gambelii Nutt.) were used to compare sensitivity of radial growth to regional drought and temperature among co‐occurring species at the same site, and between sites that differed in elevation and species composition. Results For Picea engelmannii, Pinus flexilis, Pinus ponderosa and Pseudotsuga menziesii, trees in drier, low‐elevation stands generally had greater sensitivity of radial growth to regional drought than trees of the same species in wetter, high‐elevation stands. Species low in their elevational range had greater drought sensitivity than co‐occurring species high in their elevational range at the pinyon‐juniper/ponderosa pine forest ecotone, ponderosa pine/mixed conifer forest ecotone and high‐elevation invaded meadows, but not at the mixed conifer/subalpine forest ecotone. Sensitivity of radial growth to regional drought was greater at drier, low‐elevation compared with wetter, high‐elevation forests. Yearly growth was positively correlated with measures of regional water availability at all sites, except high‐elevation invaded meadows where growth was weakly correlated with all climatic factors. Yearly growth in high‐elevation forests up to 3300 m a.s.l. was more strongly correlated with water availability than temperature. Main conclusions Severe regional drought reduced growth of all dominant tree species over a gradient of precipitation and temperature represented by a 1500‐m change in elevation, but response to drought varied between species and stands. Growth was reduced the most in drier, low‐elevation forests and in species growing low in their elevational range in ecotones, and the least for trees that had recently invaded high‐elevation meadows. Constraints on tree growth from drought and high temperature are important for high‐elevation subalpine forests located near the southern‐most range of the dominant species.     

SHOOT STRUCTURAL EFFECTS ON NEEDLE TEMPERATURES AND PHOTOSYNTHESIS IN CONIFERS

Greater needle packing (number of needles per unit stem length) among shoots of Picea engelmannii (Parry ex. Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.) collected at sun-exposed locations resulted in leaf temperatures (T₁) that were well above air temperature (T_a) and which were closer to optimum for maximum photosynthesis under field conditions. Maximum photosynthesis in these species occurred at leaf temperatures well above mean maximum T_a during most of the summer growth period. An approximate doubling in needle packing on sun shoots of these species, however, led to an estimated temperature enhancement of daily photosynthetic carbon gain of 21 to 36% compared to estimations assuming T₁ = T_a for the entire day. Elevated T₁ due to needle packing may lead to substantial increases in summer carbon gain for conifers experiencing cold-limited, short growth seasons.     

Spatial patterns in soil organic matter dynamics are shaped by mycorrhizosphere interactions in a treeline forest

Aims

In the Swedish sub-Arctic, mountain birch (Betula pubescens ssp. czerepanovii) forests mediate rapid soil C cycling relative to adjacent tundra heaths, but little is known about the role of individual trees within forests. Here we investigate the spatial extent over which trees influence soil processes.

Methods

We measured respiration, soil C stocks, root and mycorrhizal productivity and fungi:bacteria ratios at fine spatial scales along 3 m transects extending radially from mountain birch trees in a sub-Arctic ecotone forest. Root and mycorrhizal productivity was quantified using in-growth techniques and fungi:bacteria ratios were determined by qPCR.

Results

Neither respiration, nor root and mycorrhizal production, varied along transects. Fungi:bacteria ratios, soil organic C stocks and standing litter declined with increasing distance from trees.

Conclusions

As 3 m is half the average size of forest gaps, these findings suggest that forest soil environments are efficiently explored by roots and associated mycorrhizal networks of B. pubescens. Individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected. However, overall rates of soil C accumulation do vary with distance from trees, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration.

     

Growth and morphological responses to water level and nutrient supply in three emergent macrophyte species

Sanjiang Plain is the largest freshwater marsh in China, where plant zonation along water-level gradients is a common phenomenon. The aim of this experiment was to identify the role of water level and nutrient availability on plant zonation in the plain. Growth and root morphology of three perennial emergent macrophyte species were investigated by growing in two water levels (0.1 and 10.0 cm, relative to soil surface) and in two levels of nutrient supply (0 and 0.5 g slow-release fertilizer per container). In the plain, Carex lasiocarpa typically occurs at low elevations, Glyceria spiculosa at medial elevations, and Deyeuxia angustifolia at high elevations. The relative growth rate was the highest in C. lasiocarpa and the lowest in D. angustifolia in the 10.0-cm water level. Among the three species, only total biomass of D. angustifolia was affected by water level, and decreased with increasing water level. High nutrient supply led to increased total biomass in C. lasiocarpa and G. spiculosa. High water level led to an increased root diameter in G. spiculosa and a decreased root length in C. lasiocarpa. In the 10.0-cm water level, low nutrient supply led to thinner roots in D. angustifolia, but resulted in an increased specific root length (SRL) in C. lasiocarpa and root diameter in G. spiculosa. Water-level effect on root porosity was only observed in G. spiculosa, and nutrient amendment did not influence root porosity in all the species. These data indicate that both nutrient and water level are important factors regulating plant distribution pattern in the Sanjiang Plain, because both C. lasiocarpa and G. spiculosa are relatively sensitive to nutrient supply whereas D. angustifolia is sensitive to water level. Handling editor: S. M. Thomaz     

New species of ceratocystis from conifers

Summary Five new species ofCeratocystis wood staining fungi are described. The speciesCeratocystis abiocarpa is common in recently killed subalpine fir (Abies lasiocarpa (Hook)Nutt.), trees or felled logs in the Central Rocky Mountain area from Wyoming to Northern Arizona and New Mexico. It also occurs commonly in Engelmann spruce (Picea engelmannii Perry) logs infested withIps sp. bark beetles in this same area. C. minuta-bicolor is widespread in beetle infested logs of spruce, firs, pines and other conifers.C. nigrocarpa was isolated only occasionally but is apparently rather widespread in bark beetle infested pine logs.C. leucocarpa was isolated only occasionally from beetle infested pine logs and rarely in other conifers. It has been encountered mainly in the Fort Collins, Colorado area and from southern Oregon.The fifth new species (C. seticollis) was isolated only once from ambrosia beetle galleries in sapwood of a hemlock stump in eastern New York state.This study was supported by a grant from the National Science Foundation (GB-2407).     

Ultraviolet-B and visible light penetration into needles of two species of subalpine conifers during foliar development

The depth of penetration of Ultraviolet-B (UV-B, 300 and 320 nm) and visible (680 nm) light was measured in foliage of Abies lasiocarpa and Picea engelmannii using a fibre-optic microprobe. Measurements were made on foliage at four times during development: needles were sampled from within expanding buds (in bud); within 72 h of emergence from the bud scales (emergent); from elongating branches (elongating); and from foliage that emerged the previous summer (mature). Light attenuation in pre-emergent needles of both species was steep and showed strong wavelength dependence. Short wavelength 300-nm light was attenuated strongly in the developing epidermal layer, but a significant proportion of this potentially damaging UV-B radiation penetrated into the mesophyll. For A. lasiocarpa and P. engelmannii, 99% attenuation of 300-nm light occurred at 51 and 96 μm, respectively, well within the mesophyll. At this stage, however, the bud scales were opaque to light below 400nm. As the epidermal cell walls and cuticle continued to develop and chlorophyll accumulated following emergence from the bud scales, light attenuation, particularly of UV-B radiation, increased. Although no UV-B is transmitted through the epidermis-hypodermis of mature needles, small but measurable quantities of 300- and 320-nm light were measured in the photosynthetic mesophyll of post-emergent and elongating needles. Thus, shortly after emergence from the bud scales in mid-June to mid-July, when incident UV doses are highest, absorption of UV-B radiation by potentially sensitive chromophores in the mesophyll may disrupt physiological and developmental processes in these species. Soluble UV-absorbing pigments accumulated during needle maturation for P. engelmannii but not A. lasiocarpa, suggesting that, for A. lasiocarpa at least, the development of effective UV screening properties in the epidermis may not be related to the induction of soluble flavonoids.     

Germination as a determinant of seedling distributions among natural substrates in Picea engelmannii (Pinaceae) and Abies lasiocarpa (Pinaceae)

In Rocky Mountain (USA) subalpine forests, seedlings of Picea engelmannii (Engelmann spruce) colonize logs more frequently than seedlings of its codominant associate Abies lasiocarpa (subalpine fir). We hypothesized that spruce germinates more readily on logs than fir, perhaps because small spruce seeds are more likely to lodge in log crevices than larger fir seeds. Our objectives were to test this hypothesis and compare both species' germination among several substrates to assess germination influences on natural seedling distributions. Spruce and fir seeds were sown on field-collected logs, litter, and soil in the greenhouse and monitored for 36 d. To test the crevice hypothesis, seeds were either scattered on logs or wedged into crevices, assuming that if both species were placed in crevices, interspecific germination differences on logs would decrease. Spruce mean germination percentages were significantly greater than fir's in all substrate treatments except when seeds were wedged in log crevices. The difference in means between the two log treatments was greater for fir (68%) than spruce (21%). Spruce germinated more rapidly than fir on all substrates. We suggest that large seed size reduces fir's success in colonizing logs, and that germination and establishment factors interact to determine natural seedling distributions for these subalpine conifers.     

Annual soil respiration in broadleaf forests of northern Wisconsin: influence of moisture and site biological,chemical, and physical characteristics

Soil temperature and moisture influence soil respiration at a range of temporal and spatial scales. Although soil temperature and moisture may be seasonally correlated, intra and inter-annual variations in soil moisture do occur. There are few direct observations of the influence of local variation in species composition or other stand/site characteristics on seasonal and annual variations in soil moisture, and on cumulative annual soil carbon release. Soil climate and soil respiration from twelve sites in five different forest types were monitored over a 2-year period (1998–1999). Also measured were stand age, species composition, basal area, litter inputs, total above-ground wood production, leaf area index, forest floor mass, coarse and fine root mass, forest floor carbon and nitrogen concentration, root carbon and nitrogen concentration, soil carbon and nitrogen concentration, coarse fraction mass and volume, and soil texture. General soil respiration models were developed using soil temperature, daily soil moisture, and various site/soil characteristics. Of the site/soil characteristics, above-ground production, soil texture, roots + forest floor mass, roots + forest floor carbon:nitrogen, and soil carbon:nitrogen were significant predictors of soil respiration when used alone in respiration models; all of these site variables were weakly to moderately correlated with mean site soil moisture. Daily soil climate data were used to estimate the annual release of carbon (C) from soil respiration for the period 1998–1999. Mean annual soil temperature did not differ between the 2 years but mean annual soil moisture was approximately 9% lower in 1998 due to a summer drought. Soil C respired during 1998 ranged from 8.57 to 11.43 Mg C ha⁻¹ yr⁻¹ while the same sites released 10.13 and 13.57 Mg C ha⁻¹ yr⁻¹ in 1999; inter-annual differences of 15.41 and 15.73%, respectively. Among the 12 sites studied, we calculated that the depression of soil respiration linked to the drought caused annual differences of soil respiration from 11.00 to 15.78%. Annual estimates of respired soil C decreased with increasing site mean soil moisture. Similarly, the difference of respired carbon between the drought and the non-drought years generally decreased with increasing site mean soil moisture.     

Conifer seedling distribution and survival in an alpine-treeline ecotone

The importance of seedling establishment to the position ofalpine-treeline is recognized, yet little is known about factorsaffecting the survival of seedlings of treeline conifers during their initialyears of growth and establishment. This establishment period may have thegreatest mortality of all life stages until death of mature trees by disease orfire. Spatial and temporal patterns in the distribution and survival ofseedlings of Picea engelmannii and Abieslasiocarpa were evaluated over four years in analpine-treeline ecotone of the Snowy Range, Wyoming, USA. Seedlings andsaplings of both species occurred most frequently near islands of adult trees.For P. englemannii, this appeared partly due to decreasedsurvivorship of young seedlings (< 5 cm height) with greaterdistance away from tree islands. Survival of emergents of P.engelmannii was 28% greater on the north compared to southsides of tree islands, 48% lower on south-facing slopes comparedto other aspects, and 70% greater with overhead cover such as treebranches. Survival of emergents was greater in microsites with grass cover(90% survival) compared to without ground cover (44% survival),but lowest in microsites surrounded, but not covered, by grass (19%).From 1994–1999, natural seedling emergence and survival washighest in 1995 (80% survival of 221 P.engelmannii,and 100% of seven A. lasiocarpa, in a sample areaof432 m²), when the smallest mean difference in dailymaximum and minimum temperatures occurred, and lowest in 1994 (30% ofseven P. engelmannii), when above-averagetemperatures were accompanied by low rainfall and clear skies. The growthseasons of 1994 and 1995 had among the lowest and highest precipitation of theprevious 30-year period, respectively. In an artificial seedingexperiment, less than 20% of seedlings of both species survived theirfirst complete year of growth. In the autumn of the second year, almost25% additional mortality was observed in the remaining experimentalseedlings when they were exposed to clear, cold skies without the normalprotection of snowcover. Both spatial and temporal patterns of seedlingsurvivalsuggest that exposure to high sunlight may exacerbate low-temperatureandwater stress in young conifer seedlings, inhibiting their establishment in thisalpine treeline.     

The co-occurrence of ectomycorrhizal,arbuscular mycorrhizal,and dark septate fungi in seedlings of four members of the Pinaceae

Although roots of species in the Pinaceae are usually colonized by ectomycorrhizal (EM) fungi, there are increasing reports of the presence of arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi in these species. The objective of this study was to determine the colonization patterns in seedlings of three Pinus (pine) species (Pinus banksiana, Pinus strobus, Pinus contorta) and Picea glauca x Picea engelmannii (hybrid spruce) grown in soil collected from a disturbed forest site. Seedlings of all three pine species and hybrid spruce became colonized by EM, AM, and DSE fungi. The dominant EM morphotype belonged to the E-strain category; limited colonization by a Tuber sp. was found on roots of Pinus strobus and an unknown morphotype (cf. Suillus–Rhizopogon group) with thick, cottony white mycelium was present on short roots of all species. The three fungal categories tended to occupy different niches in a single root system. No correlation was found between the percent root colonized by EM and percent colonization by either AM or DSE, although there was a positive correlation between percent root length colonized by AM and DSE. Hyphae and vesicles were the only AM intracellular structures found in roots of all species; arbuscules were not observed in any roots.     

Soil microbial communities from an elevational cline differ in their effect on conifer seedling growth

Sub-alpine environments consist of altitudinal gradients associated with dramatic changes in plant growth and community composition, but the role of soil feedbacks and microbe interactions is largely unknown. Here, we examine the influence of the overall soil microbial community, with a focus on ectomycorrhizal and dark septate endophytic root colonizing fungi, from low, mid, and high elevations on the growth of Pinus contorta and Picea glauca × engelmannii. The influence of the soil microbial community was tested on seedlings from the same three elevations in order to determine ‘home’ versus ‘away’ effects on conspecifics of differing elevations. The low elevation soil was the most fertile and harbored a soil microbial community with an overall negative effect on seedling growth. In contrast, the high elevation soil was the least fertile and had a microbial community that enhanced seedling growth. However, only the soil microbial community in the highest elevation soil resulted in a stronger influence on the native P. contorta seedlings than seedlings originating from lower elevations. Despite the overall influence of the soil microbial community, ectomycorrhizal colonization was significantly correlated with P. glauca × engelmannii growth rates, but colonization by dark septate endophytes showed no relationship with seedling growth. The results provide evidence that plant—soil microbial community relationships are dependent on soil environment. Moreover, our results provide further support for the importance of soil microbes in facilitating seedling growth toward the edge of their elevational range.     

On the relative roles of hydrology, salinity, temperature, and root productivity in controlling soil respiration from coastal swamps (freshwater)

Background and aims

Soil CO₂ emissions can dominate gaseous carbon losses from forested wetlands (swamps), especially those positioned in coastal environments. Understanding the varied roles of hydroperiod, salinity, temperature, and root productivity on soil respiration is important in discerning how carbon balances may shift as freshwater swamps retreat inland with sea-level rise and salinity incursion, and convert to mixed communities with marsh plants.

Methods

We exposed soil mesocosms to combinations of permanent flooding, tide, and salinity, and tracked soil respiration over 2½ growing seasons. We also related these measurements to rates from field sites along the lower Savannah River, Georgia, USA. Soil temperature and root productivity were assessed simultaneously for both experiments.

Results

Soil respiration from mesocosms (22.7–1678.2 mg CO₂ m^?2 h^?1) differed significantly among treatments during four of the seven sampling intervals, where permanently flooded treatments contributed to low rates of soil respiration and tidally flooded treatments sometimes contributed to higher rates. Permanent flooding reduced the overall capacity for soil respiration as soils warmed. Salinity did reduce soil respiration at times in tidal treatments, indicating that salinity may affect the amount of CO₂ respired with tide more strongly than under permanent flooding. However, soil respiration related greatest to root biomass (mesocosm) and standing root length (field); any stress reducing root productivity (incl. salinity and permanent flooding) therefore reduces soil respiration.

Conclusions

Overall, we hypothesized a stronger, direct role for salinity on soil respiration, and found that salinity effects were being masked by varied capacities for increases in respiration with soil warming as dictated by hydrology, and the indirect influence that salinity can have on plant productivity.     

Forest vegetation of the Colorado Front Range

The forest vegetation of the northern Colorado Front Range was studied using a combination of gradient analysis and classification methods. A graphical model of forest composition based on elevation and topographic-moisture gradients was constructed using 305 0.1 ha samples. To derive the topographic moisture gradient, stands were stratified into eight 200 m elevation belts, and then ordinated by correspondence analysis using understory (<1 m) data. Each of the resultant gradients was scaled against a standard site moisture scalar derived from incident solar radiation and topographic position. Except for krummholz sites, the vegetation defined gradients fit the moisture scalar closely. Once scaled, these gradients were stacked vertically, sandwich-style, to create the graphical representation shown in Figure 5.Gradient analysis and ordination (direct and indirect gradient analysis of Whittaker, 1967) are frequently viewed as alternative approaches for analysis of vegetation. With gradient analysis the axes are readily interpretable, but stand placement is often difficult and at times questionable. Ordination defines an optimal arrangement for species and/or stands, but axis interpretation is often impossible. With the present combination of methods, the interpretability of gradient analysis complements the precision of placement obtained with ordination.Forest vegetation was classified by dividing the gradient model into eight series and 29 types on the basis of similar successional trends in canopy dominants. On dry, low-elevation sites above 1 700 m Pinus ponderosa woodlands dominate. With increasing elevation or site moisture, tree density increases and Pinus ponderosa, Pseudotsuga forests prevail. At middle elevations on mesic sites forests of mixed composition occur. Pinus contorta forests dominate at middle elevations over much of the central position of the moisture gradient, though these are primarily post-fire forests. With protection from fire only a small percentage of sites retain dominance by Pinus contorta. Over the lower portion of its range Pinus contorta is succeeded by Pseudotsuga, while at higher elevations Abies lasiocarpa and Picea engelmannii can eventually achieve dominance. At high elevations on all except the driest sites Picea engelmannii and Abies lasiocarpa are exclusive dominants, both after disturbance and in climax forests. Pinus flexilis dominates on the driest high-elevation sites. Above 3 500 m forests are replaced by alpine tundra, often with a transitional krummholz zone.Structure and post-fire development were examined in the context of the gradient-based classification scheme. Three generalized types of forest development were recognized as reference points in a continuum of developmental patterns varying with both elevation and soil moisture.On favorable, middle-elevation sites, trees become established rapidly after disturbance. Rapid growth results in severe overcrowding and competitive elimination of reproduction. As a consequence bell-shaped diameter distributions develop. Diversity and productivity appear to drop while biomass remains roughly constant. Following decades or even centuries of stagnation, the forests eventually breakup through mortality of the canopy trees, thereby allowing regeneration to resume. During this period of renewed regeneration, biomass, diversity, and productivity all show dramatic changes in response to the changing population structure (Fig. 9). This type of forest development can be found in forests dominated by Picea engelmannii and Abies lasiocarpa, Pinus contorta, Pseudotsuga menzeisii, Pinus flexilis or Populus tremuloides. On highest elevation forest sites or at middle elevations on the very driest sites reestablishment rates are greatly reduced. These forests dominated by Picea and Abies or Pinus flexilis gradually approach predisturbance levels of biomass, diversity and productivity, while regeneration remains at a roughly constant level. At lower elevations in the Pinus ponderosa woodlands, regeneration appears episodic, reflecting variation in seed rain and favorable conditions for seedling growth. Here, inter-tree competition is relatively unimportant and diameter distributions show irregular humps resulting from periodic recruitment.A few species pairs presented consistent problems and their treatment as single species was necessary. Garex rossii and C. brevipes were lumped as Carex rossii. Rosa woodsii and R. acicularis were lumped as Rosa sp. Cirsium scopulorum and C. coloradense were lumped as Cirsium coloradense. Extreme forms of Arnica cordifolia and A. latifolia are easily distinguishable, but as these species intergrade and hybridize extensively, they have been lumped as Arnica cordifolia. The native bluegrass, Poa agassizensis, was lumped with Poa paratensis. Solidago missouriensis includes some S. canadensis.Nomenclature follows Weber (1972) for most species. In some cases where Weber's narrow generic concept deviates from the main thrust of present-day North American systematic botany, names were changed to conform with Harrington (1954) and Hitchcock & Cronquist (1973). Voucher specimens have been deposited in the herbarium of Rocky Mountain National Park, with a few unusual species being deposited in the herbarium of the University of Colorado, Boulder.Numerous individuals have contributed generously to this project. Among those to whom I am particularly indebted are B. Chabot, R. T. Clausen, C. V. Cogbill, J. Douglas, H. G. Gauch. Jr., D. C. Glenn-Lewin, D. Hamilton, K. H. Hildebrandt, D. Mueller-Dombois, R. L. Peet, D. Stevens, E. L. Stone, J. Vleck, W. A. Weber, T. R. Wentworth, and P. L. Whittaker. I especially thank R. H. Whittaker for advice and encouragement. Financial support was provided by grants from the National Science Foundation, the DuPont Foundation, Cornell University and the University of North Carolina Research Council. The cooperation and support of the National Park Service is gratefully acknowledged.     

昌岭山两个优势树种径向生长对气候变化的响应

在气候变暖背景下,树木径向生长对气候变化的响应存在不稳定性。利用采自祁连山东部余脉昌岭山两个优势树种油松和青海云杉的树轮样芯,建立树轮宽度标准年表,通过分析树轮宽度年表与气候要素的相关关系,探讨两个树种径向生长对气候变化的响应。结果表明：（1）油松年表比青海云杉年表包含更多的气候信息,其平均敏感度、标准差、信噪比和样本对总体的代表性等统计量均高于青海云杉标准年表。（2）气候要素对不同树种径向生长限制程度不同,油松径向生长主要与降水（前一年9月和当年3-8月）和气温（前一年9月）有关,但对降水的响应更为敏感,而青海云杉径向生长则受到气温（当年9月）和降水（前一年9月、当年3月和7月）的共同作用。（3）气温突变后,油松和青海云杉年表与各气温要素的相关性显著增强,而青海云杉年表与气温要素的相关性变化更明显,指示了青海云杉径向生长对气温的响应更不稳定。（4）生长季平均最低气温的升高诱导的干旱胁迫是油松和青海云杉树木径向生长-气温响应变化的主要原因。     

Forest soil respiration rate and δ13C is regulated by recent above ground weather conditions

Soil respiration, a key component of the global carbon cycle, is a major source of uncertainty when estimating terrestrial carbon budgets at ecosystem and higher levels. Rates of soil and root respiration are assumed to be dependent on soil temperature and soil moisture yet these factors often barely explain half the seasonal variation in soil respiration. We here found that soil moisture (range 16.5–27.6% of dry weight) and soil temperature (range 8–17.5°C) together explained 55% of the variance (cross-validated explained variance; Q²) in soil respiration rate (range 1.0–3.4 mol C m^–2 s^–1) in a Norway spruce (Picea abies) forest. We hypothesised that this was due to that the two components of soil respiration, root respiration and decomposition, are governed by different factors. We therefore applied PLS (partial least squares regression) multivariate modelling in which we, together with below ground temperature and soil moisture, used the recent above ground air temperature and air humidity (vapour pressure deficit, VPD) conditions as x-variables. We found that air temperature and VPD data collected 1–4 days before respiration measurements explained 86% of the seasonal variation in the rate of soil respiration. The addition of soil moisture and soil temperature to the PLS-models increased the Q² to 93%. ¹³C analysis of soil respiration supported the hypotheses that there was a fast flux of photosynthates to root respiration and a dependence on recent above ground weather conditions. Taken together, our results suggest that shoot activities the preceding 1–6 days influence, to a large degree, the rate of root and soil respiration. We propose this above ground influence on soil respiration to be proportionally largest in the middle of the growing season and in situations when there is large day-to-day shifts in the above ground weather conditions. During such conditions soil temperature may not exert the major control on root respiration.     

Characteristics of Mount Graham Red Squirrel Nest Sites in a Mixed Conifer Forest

Abstract: The Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is constrained to the Pinaleño Mountains in southeastern Arizona, USA. The population's endangered status and extensive forest damage from insects and fire warrants a better understanding of habitat variables important for nest site selection. We examined characteristics of cavity (n = 91) and drey (n = 38) nests and compared these to random sites (n = 113). Dreys were found primarily in Engelmann spruce (Picea engelmannii) and corkbark fir (Abies lasiocarpa var. arizonica). Cavity nests occurred primarily in aspen (Populus tremuloides) and corkbark fir. Squirrels selected nest sites with higher canopy cover and more corkbark fir, decayed logs, and living trees. Forest management plans emphasizing thinning must consider how altering these habitat characteristics could affect availability and suitability of tree stands for nesting squirrels.     

Coupling of canopy gas exchange with root and rhizosphere respiration in a semi-arid forest

The strength of coupling between canopy gas exchange and root respiration was examined in ~15-yr-old ponderosa pine (Pinus ponderosa Doug. Ex Laws.) growing under seasonally drought stressed conditions. By regularly watering part of the root system to reduce tree water stress and measuring soil CO₂ efflux on the dry, distant side of the tree, we were able to determine the strength of the relationship between soil autotrophic (root and rhizosphere) respiration and changes in canopy carbon uptake and water loss by comparison with control trees (no watering). After ~40 days the soil CO₂ efflux rate, relative to pre-treatment conditions, was twice that of the controls. This difference, attributable to root and rhizosphere respiration, was strongly correlated with differences in transpiration rates between treatments (r² = 0.73, p<0.01). By the end of the period, transpiration of the irrigated treatment was twice that of controls. Periodic measurements of photosynthesis under non-light limited conditions paralleled the patterns of transpiration and were systematically higher in the irrigated treatment. We observed no evidence for a greater sensitivity of soil autotrophic respiration to temperature compared to the response of heterotrophic respiration to temperature; the Q₁₀ for total soil respiration was 1.6 (p>0.99) for both treatments. At the ecosystem scale, daily soil CO₂ efflux rate was linearly related to gross primary productivity (GPP) as measured by eddy-covariance technique (r² = 0.55, p<0.01), suggesting patterns of soil CO₂ release appear strongly correlated to recent carbon assimilation in this young pine stand. Collectively the observed relationships suggest some consideration should be given to the inclusion of canopy processes in future models of soil respiration.     

The mountain forests of british columbia and the american northwest: Floristic patterns and syntaxonomy

An overview of the montane and subalpine forests ofAbies lasiocarpa, Picea engelmannii, Abies amabilis andTsuga mertensiana of northwestern North America is provided based on 541 relevés of forest vegetation from coastal and inland British Columbia, Washington and northwestern Montana. A new classification is proposed based on an examination of the floristic patterns across these communities. Owing to their broadscale floristic similarities, coastal subalpineAbies amabilis-Tsuga mertensiana forests and inland forests ofAbies lasiocarpa, Picea engelmannii, P. glauca, Thuja plicata andTsuga heterophylla are combined into a single order,Thujetalia plicatae. The floristic patterns within this order in turn support the recognition of four alliances. These are (1)Tsugion mertensianae, including coastal and inland subalpine forests ofTsuga mertensiana, Abies amabilis, A. lasiocarpa andPicea engelmannii, with deep winter snowpacks and an often luxuriously developed shrub layer; (2)Abieti-Tsugion heterophyllae all. nov., including mesic inland montaneThuja plicata-Tsuga heterophylla andAbies lasiocarpa-Picea engelmannii forests and related southern borealAbies lasiocarpa-Picea glauca communities; (3)Gymnocarpio-Abietion lasiocarpae, including moist coastal and inland forests dominated by a fern-rich understory andOplopanax horridus; and (4)Lysichito-Chamaecyparidion nootkatensis, which includes wet coastal subalpine forests withLysichiton americanum. A total of five new associations are described, bringing the number of recognized associations within the order to twenty-five. An extensive synonymy and crosswalk to other classification systems is provided.